High-strength steel sheet, high-strength hot-dip galvanized steel sheet, high-strength hot-dip aluminum-coated steel sheet, and high-strength electrogalvanized steel sheet, and methods for manufacturing same

ABSTRACT

A high-strength steel sheet with excellent formability and high yield ratio that has TS of 590 MPa or more and YR of 68% or more is obtained by providing a predetermined chemical composition and a steel microstructure that contains, in area ratio, 35 to 80% of polygonal ferrite, 5% or more of non-recrystallized ferrite, and 5 to 25% of martensite, and that contains, in volume fraction, 8% or more of retained austenite, in which the polygonal ferrite has a mean grain size of 6 μm or less, the martensite has a mean grain size of 3 μm or less, the retained austenite has a mean grain size of 3 μm or less, and a value obtained by dividing an Mn content in the retained austenite (in mass %) by an Mn content in the polygonal ferrite (in mass %) equals 2.0 or more.

TECHNICAL FIELD

This disclosure relates to a high-strength steel sheet, a high-strengthhot-dip galvanized steel sheet, a high-strength hot-dip aluminum-coatedsteel sheet, and a high-strength electrogalvanized steel sheet, andmethods for manufacturing the same, and in particular to, the provisionof a steel sheet with excellent formability and high yield ratio that ispreferably used in parts in the industrial fields of automobiles,electronics, and the like.

BACKGROUND

In recent years, enhancement of fuel efficiency of automobiles hasbecome an important issue from the viewpoint of global environmentprotection. Consequently, there is an active movement to reduce thethickness of vehicle body components through increases in strength ofvehicle body materials, and thereby reduce the weight of vehicle bodyitself.

In general, however, strengthening of steel sheets leads todeterioration in formability, causing the problem of cracking duringforming. It is thus not simple to reduce the thickness of steel sheets.Therefore, it is desirable to develop materials with increased strengthand good formability. In addition to good formability, steel sheets witha tensile strength (TS) of 590 MPa or more are required to have, inparticular, enhanced impact energy absorption properties. To enhanceimpact energy absorption properties, it is effective to increase yieldratio (YR). The reason is that a higher yield ratio enables the steelsheet to absorb impact energy more effectively with less deformation.

For example, JPS61157625A (PTL 1) proposes a high-strength steel sheetwith extremely high ductility having a tensile strength of 1000 MPa orhigher and a total elongation (EL) of 30% or more, utilizing deformationinduced transformation of retained austenite.

In addition, JPH1259120A (PTL 2) proposes a high-strength steel sheetwith well-balanced strength and ductility that is obtained from high-Mnsteel through heat treatment in a ferrite-austenite dual phase region.

Moreover, JP2003138345A (PTL 3) proposes a high-strength steel sheetwith improved local ductility that is obtained from high-Mn steelthrough hot rolling to have a microstructure containing bainite andmartensite after subjection to the hot rolling, followed by annealingand tempering to cause fine retained austenite, and subsequentlytempered bainite or tempered martensite in the microstructure.

CITATION LIST Patent Literature

PTL 1: JPS61157625A

PTL 2: JPH1259120A

PTL 3: JP2003138345A

SUMMARY Technical Problem

The steel sheet described in PTL 1 is manufactured by austenitizing asteel sheet containing C, Si, and Mn as basic components, and subjectingthe steel sheet to a so-called austempering process whereby the steelsheet is quenched to and held isothermally in a bainite transformationtemperature range. During the austempering process, C concentrates inaustenite to form retained austenite.

However, a high concentration of C beyond 0.3% is required for theformation of a large amount of retained austenite, such a high Cconcentration above 0.3% leads to a significant decrease in spotweldability, which may not be suitable for practical use in steel sheetsfor automobiles.

Additionally, the main objective of PTL 1 is improving the ductility ofsteel sheets, without any consideration for the hole expansionformability, bendability, or yield ratio.

PTLs 2 and 3 describes techniques for improving the ductility of steelsheets from the perspective of formability, but do not consider thebendability or yield ratio of the steel sheet.

To address these issues, it could thus be helpful to provide ahigh-strength steel sheet, a high-strength hot-dip galvanized steelsheet, a high-strength hot-dip aluminum-coated steel sheet, and ahigh-strength electrogalvanized steel sheet that are excellent informability with TS of 590 MPa or more and YR of 68% or more, andmethods for manufacturing the same.

Solution to Problem

To manufacture a high-strength steel sheet that can address the aboveissues, with excellent formability as well as high yield ratio and hightensile strength, we made intensive studies from the perspectives of thechemical compositions and manufacturing methods of steel sheets. As aresult, we discovered that a high-strength steel sheet with high yieldratio that is excellent in formability such as ductility can bemanufactured by appropriately controlling the chemical composition andmicrostructure of steel.

Specifically, a steel sheet that has a steel composition containing 2.60mass % to 4.20 mass % of Mn, with the addition amounts of other alloyingelements such as Ti being adjusted appropriately, is hot rolled toobtain a hot-rolled sheet. The hot-rolled sheet is then subjected topickling to remove scales, retained in a temperature range of Ac₁transformation temperature+20° C. to Ac₁ transformation temperature+120°C. for 600 s to 21,600 s, and cold rolled at a rolling reduction of 30%or more to obtain a cold-rolled sheet. Further, the cold-rolled sheet isretained in a temperature range of Ac₁ transformation temperature to Ac₁transformation temperature+100° C. for 20 s to 900 s, and subsequentlycooled.

Through this process, the cold-rolled sheet has a microstructure thatcontains, in area ratio, 35% or more and 80% or less of polygonalferrite, 5% or more of non-recrystallized ferrite, and 5% or more and25% or less of martensite, where the polygonal ferrite has a mean grainsize of 6 μm or less, the martensite has a mean grain size of 3 μm orless, and the retained austenite has a mean grain size of 3 μm or less.Moreover, the microstructure of the cold-rolled sheet can be controlledso that a value obtained by dividing an Mn content in the retainedaustenite (in mass %) by an Mn content in the polygonal ferrite (in mass%) equals 2.0 or more, making it possible to obtain retained austenitestabilized with Mn in an amount of 8% or more.

This disclosure has been made based on these discoveries.

Specifically, the primary features of this disclosure are as describedbelow.

[1] A high-strength steel sheet comprising: a chemical compositioncontaining (consisting of), in mass %, C: 0.030% or more and 0.250% orless, Si: 0.01% or more and 3.00% or less, Mn: 2.60% or more and 4.20%or less, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and0.0200% or less, N: 0.0005% or more and 0.0100% or less, and Ti: 0.005%or more and 0.200% or less, and the balance consisting of Fe andincidental impurities; and a steel microstructure that contains, in arearatio, 35% or more and 80% or less of polygonal ferrite, 5% or more ofnon-recrystallized ferrite, and 5% or more and 25% or less ofmartensite, and that contains, in volume fraction, 8% or more ofretained austenite, wherein the polygonal ferrite has a mean grain sizeof 6 μm or less, the martensite has a mean grain size of 3 μm or less,the retained austenite has a mean grain size of 3 μm or less, and avalue obtained by dividing an Mn content in the retained austenite inmass % by an Mn content in the polygonal ferrite in mass % equals 2.0 ormore.

[2] The high-strength steel sheet according to [1], wherein the chemicalcomposition further contains, in mass %, at least one selected from thegroup consisting of Al: 0.01% or more and 2.00% or less, Nb: 0.005% ormore and 0.200% or less, B: 0.0003% or more and 0.0050% or less, Ni:0.005% or more and 1.000% or less, Cr: 0.005% or more and 1.000% orless, V: 0.005% or more and 0.500% or less, Mo: 0.005% or more and1.000% or less, Cu: 0.005% or more and 1.000% or less, Sn: 0.002% ormore and 0.200% or less, Sb: 0.002% or more and 0.200% or less, Ta:0.001% or more and 0.010% or less, Ca: 0.0005% or more and 0.0050% orless, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or moreand 0.0050% or less.

[3] The high-strength steel sheet according to [1] or [2], wherein theretained austenite has a C content that satisfies the following formulain relation to the Mn content in the retained austenite:

0.09*[Mn content]−0.026−0.150≦[C content]≦0.09*[Mn content]−0.026+0.150

where

[C content] is the C content in the retained austenite in mass %, and

[Mn content] is the Mn content in the retained austenite in mass %.

[4] The high-strength steel sheet according to any one of [1] to [3],wherein when the steel sheet is subjected to tensile working with anelongation value of 10%, a value obtained by dividing a volume fractionof the retained austenite after the tensile working by a volume fractionof the retained austenite before the tensile working equals 0.3 or more.

[5] The high-strength steel sheet according to any one of [1] to [4],wherein the high-strength steel sheet is a high-strength hot-dipgalvanized steel sheet comprising a hot-dip galvanized layer.

[6] The high-strength steel sheet according to any one of [1] to [4],wherein the high-strength steel sheet is a high-strength hot-dipaluminum-coated steel sheet comprising a hot-dip aluminum-coated layer.

[7] The high-strength steel sheet according to any one of [1] to [4],wherein the high-strength steel sheet is a high-strengthelectrogalvanized steel sheet comprising an electrogalvanized layer.

[8] A method for manufacturing the high-strength steel sheet as recitedin any one of [1] to [4], the method comprising: heating a steel slabhaving the chemical composition as recited in [1] or [2] to 1100° C. orhigher and 1300° C. or lower; hot rolling the steel slab with a finisherdelivery temperature of 750° C. or higher and 1000° C. or lower toobtain a steel sheet; coiling the steel sheet at 300° C. or higher and750° C. or lower; then subjecting the steel sheet to pickling to removescales; retaining the steel sheet in a temperature range of Ac₁transformation temperature+20° C. to Ac₁ transformation temperature+120°C. for 600 s to 21,600 s; cold rolling the steel sheet at a rollingreduction of 30% or more; and then retaining the steel sheet in atemperature range of Ac₁ transformation temperature to Ac₁transformation temperature+100° C. for 20 s to 900 s, and subsequentlycooling the steel sheet.

[9] A method for manufacturing the high-strength steel sheet as recitedin [5], the method comprising: heating a steel slab having the chemicalcomposition as recited in [1] or [2] to 1100° C. or higher and 1300° C.or lower; hot rolling the steel slab with a finisher deliverytemperature of 750° C. or higher and 1000° C. or lower to obtain a steelsheet; coiling the steel sheet at 300° C. or higher and 750° C. orlower; then subjecting the steel sheet to pickling to remove scales;retaining the steel sheet in a temperature range of Ac₁ transformationtemperature+20° C. to Ac₁ transformation temperature+120° C. for 600 sto 21,600 s; cold rolling the steel sheet at a rolling reduction of 30%or more; then retaining the steel sheet in a temperature range of Actransformation temperature to Ac₁ transformation temperature+100° C. for20 s to 900 s, and subsequently cooling the steel sheet; and thensubjecting the steel sheet to galvanizing treatment, either alone orfollowed by alloying treatment at 450° C. or higher and 600° C. orlower.

[10] A method for manufacturing the high-strength steel sheet as recitedin [6], the method comprising: heating a steel slab having the chemicalcomposition as recited in [1] or [2] to 1100° C. or higher and 1300° C.or lower; hot rolling the steel slab with a finisher deliverytemperature of 750° C. or higher and 1000° C. or lower to obtain a steelsheet; coiling the steel sheet at 300° C. or higher and 750° C. orlower; then subjecting the steel sheet to pickling to remove scales;retaining the steel sheet in a temperature range of Ac₁ transformationtemperature+20° C. to Ac₁ transformation temperature+120° C. for 600 sto 21,600 s; cold rolling the steel sheet at a rolling reduction of 30%or more; then retaining the steel sheet in a temperature range of Actransformation temperature to Ac₁ transformation temperature+100° C. for20 s to 900 s, and subsequently cooling the steel sheet; and thensubjecting the steel sheet to hot-dip aluminum-coating treatment.

[11] A method for manufacturing the high-strength steel sheet as recitedin [7], the method comprising: heating a steel slab having the chemicalcomposition as recited in [1] or [2] to 1100° C. or higher and 1300° C.or lower; hot rolling the steel slab with a finisher deliverytemperature of 750° C. or higher and 1000° C. or lower to obtain a steelsheet; coiling the steel sheet at 300° C. or higher and 750° C. orlower; then subjecting the steel sheet to pickling to remove scales;retaining the steel sheet in a temperature range of Ac₁ transformationtemperature+20° C. to Ac₁ transformation temperature+120° C. for 600 sto 21,600 s; cold rolling the steel sheet at a rolling reduction of 30%or more; then retaining the steel sheet in a temperature range of Ac₁transformation temperature to Ac₁ transformation temperature+100° C. for20 s to 900 s, and subsequently cooling the steel sheet; and thensubjecting the steel sheet to electrogalvanizing treatment.

Advantageous Effect

According to the disclosure, it becomes possible to provide ahigh-strength steel sheet with excellent formability and high yieldratio that exhibits TS of 590 MPa or more and YR of 68% or more.High-strength steel sheets according to the disclosure are highlybeneficial in industrial terms, because they can improve fuel efficiencywhen applied to, for example, automobile structural parts, by areduction in the weight of automotive bodies.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawings:

FIG. 1 illustrates the relationship between the working ratio of tensileworking and the amount of retained austenite; and

FIG. 2 illustrates the relationship between the elongation of each steelsheet and the value obtained by dividing the volume fraction of retainedaustenite remaining in the steel sheet after subjection to tensileworking with an elongation value of 10% by the volume fraction ofretained austenite before the tensile working.

DETAILED DESCRIPTION

The following describes the present disclosure in detail.

First, the reasons for limiting the chemical composition of the steel tothe aforementioned ranges in the present disclosure are explained. The %representations below indicating the chemical composition of the steelor steel slab are in mass % unless stated otherwise. The balance of thechemical composition of the steel or steel slab consists of Fe andincidental impurities.

C: 0.030% or More and 0.250% or Less

C is an element necessary for causing a low-temperature transformationphase such as martensite to increase strength. C is also a usefulelement for increasing the stability of retained austenite and theductility of steel. If the C content is less than 0.030%, it isdifficult to ensure a desired area ratio of martensite, and desiredstrength is not obtained. It is also difficult to guarantee a sufficientvolume fraction of retained austenite, and good ductility is notobtained. On the other hand, if C is excessively added to the steelbeyond 0.250%, hard martensite excessively increases in area ratio,which causes more microvoids at grain boundaries of martensite andfacilitates propagation of cracks during bend test and hole expansiontest, leading to a reduction in bendability and stretch flangeability.If excessive C is added to steel, hardening of welds and theheat-affected zone (HAZ) becomes significant and the mechanicalproperties of the welds deteriorate, leading to a reduction in spotweldability, arc weldability, and the like. From these perspectives, theC content is 0.030% or more and 0.250% or less. The C content ispreferably 0.080% or more. The C content is preferably 0.200% or less.

Si: 0.01% or More and 3.00% or Less

Si is an element that improves the strain hardenability of ferrite, andis thus a useful element for ensuring good ductility. If the Si contentis below 0.01%, the addition effect is limited. Thus the lower limit is0.01%. On the other hand, excessively adding Si beyond 3.00% not onlyembrittles the steel, but also causes red scales or the like todeteriorate surface characteristics. Therefore, the Si content is 0.01%or more and 3.00% or less. The Si content is preferably 0.20% or more.The Si content is preferably 2.00% or less.

Mn: 2.60% or More and 4.20% or Less

Mn is one of the very important elements for the disclosure. Mn is anelement that stabilizes retained austenite, and is thus a useful elementfor ensuring good ductility. Mn can also increase TS of the steelthrough solid solution strengthening. These effects can be obtained whenthe Mn content in steel is 2.60% or more. On the other hand, excessivelyadding Mn beyond 4.20% results in a rise in cost. From theseperspectives, the Mn content is 2.60% or more and 4.20% or less. The Mncontent is preferably 3.00% or more. The Mn content is preferably 4.20%or less.

P: 0.001% or More and 0.100% or Less

P is an element that has a solid solution strengthening effect and canbe added depending on the desired TS. P also facilitates ferritetransformation, and thus is also a useful element for forming amulti-phase structure in the steel sheet. To obtain this effect, the Pcontent in the steel sheet needs to be 0.001% or more. However, if the Pcontent exceeds 0.100%, weldability degrades and, when a galvanizedlayer is subjected to alloying treatment, the alloying rate decreases,impairing galvanizing quality. Therefore, the P content is 0.001% ormore and 0.100% or less. The P content is preferably 0.005% or more. TheP content is preferably 0.050% or less.

S: 0.0001% or More and 0.0200% or Less

S segregates to grain boundaries, embrittles the steel during hotworking, and forms sulfides to reduce the local deformability of thesteel sheet. Therefore, the S content is 0.0200% or less, preferably0.0100% or less, and more preferably 0.0050% or less. Under productionconstraints, however, the S content is 0.0001% or more. Therefore, the Scontent is 0.0001% or more and 0.0200% or less. The S content ispreferably 0.0001% or more. The S content is preferably 0.0100% or less.The S content is more preferably 0.0001% or more. The S content is morepreferably 0.0050% or less.

N: 0.0005% or More and 0.0100% or Less

N is an element that deteriorates the anti-aging property of the steel.The deterioration in anti-aging property becomes more pronounced,particularly when the N content exceeds 0.0100%. Accordingly, smaller Ncontents are more preferable. However, under production constraints, theN content is 0.0005% or more. Therefore, the N content is 0.0005% ormore and 0.0100% or less. The N content is preferably 0.0010% or more.The N content is preferably 0.0070% or less.

Ti: 0.005% or More and 0.200% or Less

Ti is one of the very important elements for the disclosure. Ti isuseful for achieving strengthening by precipitation of the steel. Ti canalso ensure a desired area ratio of non-recrystallized ferrite, andcontributes to increasing the yield ratio of the steel sheet.Additionally, making use of relatively hard non-recrystallized ferrite,Ti can reduce the difference in hardness from a hard secondary phase(martensite or retained austenite), and also contributes to improvingstretch flangeability. These effects can be obtained when the Ti contentis 0.005% or more. On the other hand, if the Ti content in the steelexceeds 0.200%, hard martensite excessively increases in area ratio,which causes more microvoids at grain boundaries of martensite andfacilitates propagation of cracks during bend test and hole expansiontest, leading to a reduction in the bendability and stretchflangeability of the steel sheet. Therefore, the Ti content is 0.005% ormore and 0.200% or less. The Ti content is preferably 0.010% or more.The Ti content is preferably 0.100% or less.

The chemical composition of the steel may further contain at least oneselected from the group consisting of Al: 0.01% or more and 2.00% orless, Nb: 0.005% or more and 0.200% or less, B: 0.0003% or more and0.0050% or less, Ni: 0.005% or more and 1.000% or less, Cr: 0.005% ormore and 1.000% or less, V: 0.005% or more and 0.500% or less, Mo:0.005% or more and 1.000% or less, Cu: 0.005% or more and 1.000% orless, Sn: 0.002% or more and 0.200% or less, Sb: 0.002% or more and0.200% or less, Ta: 0.001% or more and 0.010% or less, Ca: 0.0005% ormore and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, andREM: 0.0005% or more and 0.0050% or less.

Al is a useful element for increasing the area of a ferrite-austenitedual phase region and reducing annealing temperature dependency, i.e.,increasing the stability of the steel sheet as a material. In addition,Al acts as a deoxidizer, and is also a useful element for maintainingthe cleanliness of the steel. If the Al content is below 0.01%, however,the addition effect is limited. Thus the lower limit is 0.01%. On theother hand, excessively adding Al beyond 2.00% increases the risk ofcracking occurring in a semi-finished product during continuous casting,and inhibits manufacturability. From these perspectives, the Al contentis 0.01% or more and 2.00% or less. The Al content is preferably 0.20%or more. The Al content is preferably 1.20% or less.

Nb is useful for achieving strengthening by precipitation of the steel.The addition effect can be obtained when the content is 0.005% or more.Nb can also ensure a desired area ratio of non-recrystallized ferrite,as in the case of adding Ti, and contributes to increasing the yieldratio of the steel sheet. Additionally, making use of relatively hardnon-recrystallized ferrite, Nb can reduce the difference in hardnessfrom a hard secondary phase (martensite or retained austenite), and alsocontributes to improving stretch flangeability. On the other hand, ifthe Nb content in the steel exceeds 0.200%, hard martensite excessivelyincreases in area ratio, which causes more microvoids at grainboundaries of martensite and facilitates propagation of cracks duringbend test and hole expansion test. This leads to a reduction in thebendability and stretch flangeability of the steel sheet. This alsoincreases cost. Therefore, when added to steel, the Nb content is 0.005%or more and 0.200% or less. The Nb content is preferably 0.010% or more.The Nb content is preferably 0.100% or less.

B may be added as necessary, since it has the effect of suppressing thegeneration and growth of ferrite from austenite grain boundaries andenables microstructure control according to the circumstances. Theaddition effect can be obtained when the B content is 0.0003% or more.If the B content exceeds 0.0050%, however, the formability of the steelsheet degrades. Therefore, when added to steel, the B content is 0.0003%or more and 0.0050% or less. The B content is preferably 0.0005% ormore. The B content is preferably 0.0030% or less.

Ni is an element that stabilizes retained austenite, and is thus auseful element for ensuring good ductility, and that increases TS of thesteel through solid solution strengthening. The addition effect can beobtained when the Ni content is 0.005% or more. On the other hand, ifthe Ni content in the steel exceeds 1.000%, hard martensite excessivelyincreases in area ratio, which causes more microvoids at grainboundaries of martensite and facilitates propagation of cracks duringbend test and hole expansion test. This leads to a reduction in thebendability and stretch flangeability of the steel sheet. This alsoincreases cost. Therefore, when added to steel, the Ni content is 0.005%or more and 1.000% or less.

Cr, V, and Mo are elements that may be added as necessary, since theyhave the effect of improving the balance between TS and ductility. Theaddition effect can be obtained when the Cr content is 0.005% or more,the V content is 0.005% or more, and/or the Mo content is 0.005% ormore. However, if the Cr content exceeds 1.000%, the V content exceeds0.500%, and/or the Mo content exceeds 1.000%, hard martensiteexcessively increases in area ratio, which causes more microvoids atgrain boundaries of martensite and facilitates propagation of cracksduring bend test and hole expansion test. This leads to a reduction inthe bendability and stretch flangeability of the steel sheet, and alsocauses a rise in cost. Therefore, when added to steel, the Cr content is0.005% or more and 1.000% or less, the V content is 0.005% or more and0.500% or less, and/or the Mo content is 0.005% or more and 1.000% orless.

Cu is a useful element for strengthening of steel and may be added forstrengthening of steel, as long as the content is within the rangedisclosed herein. The addition effect can be obtained when the Cucontent is 0.005% or more. On the other hand, if the Cu content in thesteel exceeds 1.000%, hard martensite excessively increases in arearatio, which causes more microvoids at grain boundaries of martensiteand facilitates propagation of cracks during bend test and holeexpansion test. This leads to a reduction in the bendability and stretchflangeability of the steel sheet. Therefore, when added to steel, the Cucontent is 0.005% or more and 1.000% or less.

Sn and Sb are elements that may be added as necessary from theperspective of suppressing decarbonization of a region extending fromthe surface layer of the steel sheet to a depth of about several tens ofmicrometers, which results from nitriding and/or oxidation of the steelsheet surface. Suppressing nitriding and/or oxidation in this way isuseful for preventing a reduction in the area ratio of martensite in thesteel sheet surface, and for ensuring the TS and stability of the steelsheet as a material. However, excessively adding Sn or Sb beyond 0.200%reduces toughness. Therefore, when Sn and/or Sb is added to steel, thecontent of each added element is 0.002% or more and 0.200% or less.

Ta forms alloy carbides or alloy carbonitrides, and contributes toincreasing the strength of the steel, as is the case with Ti and Nb. Itis also believed that Ta has the effect of effectively suppressingcoarsening of precipitates when partially dissolved in Nb carbides or Nbcarbonitrides to form complex precipitates, such as (Nb, Ta) (C, N), andproviding a stable contribution to increasing the strength of the steelsheet through strengthening by precipitation. Therefore, Ta ispreferably added to the steel according to the disclosure. The additioneffect of Ta can be obtained when the Ta content is 0.001% or more.Excessively adding Ta, however, fails to increase the addition effect,but instead results in a rise in alloying cost. Therefore, when added tosteel, the Ta content is 0.001% or more and 0.010% or less.

Ca, Mg, and REM are useful elements for causing spheroidization ofsulfides and mitigating the adverse effect of sulfides on hole expansionformability (stretch flangeability). To obtain this effect, it isnecessary to add any of these elements to steel in an amount of 0.0005%or more. However, if the content of each added element exceeds 0.0050%,more inclusions occur, for example, and some defects such as surfacedefects and internal defects are caused in the steel sheet. Therefore,when Ca, Mg, and/or REM is added to steel, the content of each addedelement is 0.0005% or more and 0.0050% or less.

The following provides a description of the microstructure.

Area Ratio of Polygonal Ferrite: 35% or More and 80% or Less

According to the disclosure, the area ratio of polygonal ferrite needsto be 35% or more to ensure sufficient ductility. On the other hand, toguarantee TS of 590 MPa or more, the area ratio of soft polygonalferrite needs to be 80% or less. The area ratio of polygonal ferrite ispreferably 40% or more. The area ratio of polygonal ferrite ispreferably 75% or less. As used herein, “polygonal ferrite” refers toferrite that is relatively soft and that has high ductility.

Area Ratio of Non-Recrystallized Ferrite: 5% or More

In this disclosure, it is very important to set the area ratio ofnon-recrystallized ferrite to be 5% or more. In this regard,non-recrystallized ferrite is useful for increasing the strength of thesteel sheet. However, non-recrystallized ferrite may cause a significantdecrease in the ductility of the steel sheet, and thus is normallyreduced in a general process. In contrast, according to the presentdisclosure, by using polygonal ferrite and retained austenite to providegood ductility and intentionally utilizing relatively hardnon-recrystallized ferrite, it is possible to provide the steel sheetwith the intended TS, without having to form a large amount ofmartensite, such as exceeding 25% in area ratio.

Moreover, according to the present disclosure, interfaces betweendifferent phases, namely, between polygonal ferrite and martensite, arereduced, making it possible to increase the yield point (YP) and YR ofthe steel sheet.

To obtain these effects, the area ratio of non-recrystallized ferriteneeds to be 5% or more, and preferably 8% or more. As used herein,“non-recrystallized ferrite” refers to ferrite that contains strain inthe grains with a crystal orientation difference of less than 15°, andthat is harder than the above-described polygonal ferrite with highductility.In the disclosure, no upper limit is placed on the area ratio ofnon-recrystallized ferrite, yet a preferred upper limit is around 45%,considering the possibility of increased material anisotropy in thesteel sheet surface.

Area Ratio of Martensite: 5% or More and 25% or Less

To achieve TS of 590 MPa or more, the area ratio of martensite needs tobe 5% or more. On the other hand, to ensure good ductility, the arearatio of martensite needs to be limited to 25% or less.According to the disclosure, the area ratios of ferrite (includingpolygonal ferrite and non-recrystallized ferrite) and martensite can bedetermined in the following way.Specifically, a cross section of a steel sheet that is taken in thesheet thickness direction to be parallel to the rolling direction (whichis an L-cross section) is polished, then etched with 3 vol. % nital, andten locations are observed at 2000 times magnification under an SEM(scanning electron microscope), at a position of sheet thickness×¼(which is the position at a depth of one-fourth of the sheet thicknessfrom the steel sheet surface), to capture microstructure micrographs.The captured microstructure micrographs are used to calculate the arearatios of respective phases (ferrite and martensite) for the tenlocations using Image-Pro manufactured by Media Cybernetics, the resultsare averaged, and each average is used as the area ratio of thecorresponding phase. In the microstructure micrographs, polygonalferrite and non-recrystallized ferrite appear as a gray structure (basesteel structure), while martensite as a white structure.

According to the disclosure, the area ratios of polygonal ferrite andnon-recrystallized ferrite can be determined in the following way.Specifically, low-angle grain boundaries in which the crystalorientation difference is from 2° to less than 15° and large-angle grainboundaries in which the crystal orientation difference is 15° or moreare identified using EBSD (Electron Backscatter Diffraction). An IQ Mapis then created, considering ferrite that contains low-angle grainboundaries in the grains as non-recrystallized ferrite. Then, low-anglegrain boundaries and large-angle grain boundaries are extracted from thecreated IQ Map at ten locations, respectively, to determine the areas oflow-angle grain boundaries and large-angle grain boundaries. Based onthe results, the areas of polygonal ferrite and non-recrystallizedferrite are calculated to determine the area ratios of polygonal ferriteand non-recrystallized ferrite for the ten locations. By averaging theresults, the above-described area ratios of polygonal ferrite andnon-recrystallized ferrite are determined.

Volume Fraction of Retained Austenite: 8% or More

According to the disclosure, the volume fraction of retained austeniteneeds to be 8% or more, preferably 10% or more, to ensure sufficientductility. According to the disclosure, no upper limit is placed on thearea ratio of retained austenite, yet a preferred upper limit is around40%, considering the risk of formation of increased amounts of unstableretained austenite resulting from insufficient concentration of C, Mn,and the like, which is less effective in improving ductility.

The volume fraction of retained austenite is calculated by determiningthe x-ray diffraction intensity of a plane of sheet thickness×¼ (whichis the plane at a depth of one-fourth of the sheet thickness from thesteel sheet surface), which is exposed by polishing the steel sheetsurface to a depth of one-fourth of the sheet thickness. Using anincident x-ray beam of MoKα, the intensity ratio of the peak integratedintensity of the {111}, {200}, {220}, and {311} planes of retainedaustenite to the peak integrated intensity of the {110}, {200}, and{211} planes of ferrite is calculated for all of the twelvecombinations, the results are averaged, and the average is used as thevolume fraction of retained austenite.

Mean Grain Size of Polygonal Ferrite: 6 μm or Less

Refinement of polygonal ferrite grains contributes to improving YP andTS. Thus, to ensure a high YP and a high YR as well as a desired TS,polygonal ferrite needs to have a mean grain size of 6 μm or less, andpreferably 5 μm or less.According to the disclosure, no lower limit is placed on the mean grainsize of polygonal ferrite, yet, from an industrial perspective, apreferred lower limit is around 0.3 μm.

Mean Grain Size of Martensite: 3 μm or Less

Refinement of martensite grains contributes to improving bendability andstretch flangeability (hole expansion formability). Thus, to ensure highbendability and high stretch flangeability (high hole expansionformability), the mean grain size of martensite needs to be limited to 3μm or less, and preferably to 2.5 μm or less.According to the disclosure, no lower limit is placed on the mean grainsize of martensite, yet, from an industrial perspective, a preferredlower limit is around 0.1 μm.

Mean Grain Size of Retained Austenite: 3 μm or Less

Refinement of retained austenite grains contributes to improvingductility, as well as bendability and stretch flangeability (holeexpansion formability). Accordingly, to ensure good ductility,bendability, and stretch flangeability (hole expansion formability) ofthe steel sheet, the mean grain size of retained austenite needs to be 3μm or less, and preferably 2.5 μm or less.According to the disclosure, no lower limit is placed on the mean grainsize of retained austenite, yet, from an industrial perspective, apreferred lower limit is around 0.1 μm.

The mean grain sizes of polygonal ferrite, martensite, and retainedaustenite are respectively determined by averaging the results fromcalculating equivalent circular diameters from the areas of polygonalferrite grains, martensite grains, and retained austenite grainsmeasured with Image-Pro as mentioned above. Martensite and retainedaustenite are identified using an EBSD phase map. In this case, each ofthe above-described mean grain sizes is determined from the measurementsfor grains with a grain size of 0.01 μm or more. The reason is thatgrains with a grain size of less than 0.01 μm have no effect on thedisclosure.

A Value Obtained by Dividing the Mn Content in the Retained Austenite(in Mass %) by the Mn Content in the Polygonal Ferrite (in Mass %): 2.0or More

In this disclosure, it is very important that the value obtained bydividing the Mn content in the retained austenite (in mass %) by the Mncontent in the polygonal ferrite (in mass %) equals 2.0 or more. Thereason is that better ductility requires a larger amount of stableretained austenite with concentrated Mn.According to the disclosure, no upper limit is placed on the valueobtained by dividing the Mn content in the retained austenite (in mass%) by the Mn content in the polygonal ferrite (in mass %), yet apreferred upper limit is around 16.0 from the perspective of ensuringstretch flangeability.

The Mn content in the retained austenite (in mass %) and the Mn contentin the polygonal ferrite (in mass %) can be determined in the followingway.

Specifically, an EPMA (Electron Probe Micro Analyzer) is used toquantify the distribution of Mn in each phase in a cross section alongthe rolling direction at a position of sheet thickness×¼. Then, 30retained austenite grains and 30 ferrite grains are analyzed todetermine respective Mn contents, the results are averaged, and eachaverage is used as the Mn content in the corresponding phase.

In addition to the above-described polygonal ferrite, martensite, and soon, the microstructure according to the disclosure further includecarbides ordinarily found in iron and steel sheets, such as granularferrite, acicular ferrite, bainitic ferrite, tempered martensite,pearlite, and cementite (excluding cementite in pearlite). Any of thesestructures may be included as long as the area ratio is 10% or less,without impairing the effect of the disclosure.

We made further investigations on the microstructures of steel sheetsupon performing press forming and working.

As a result, it was discovered that there are two types of retainedaustenite: one transforms to martensite immediately upon the subjectionof the steel sheet to press forming or working, while the other persistsuntil the working ratio becomes high enough to cause the retainedaustenite to eventually transform to martensite, bringing about a TRIPphenomenon. It was also revealed that good elongation can be obtained ina particularly effective way when a large amount of retained austenitetransforms to martensite after the working ratio becomes high enough.

Specifically, as a result of collecting samples with good and poorelongation and measuring the quantity of retained austenite by varyingthe degree of tensile working from 0% to 20%, the working ratio and thequantity of retained austenite showed a tendency as illustrated inFIG. 1. As used herein, “the working ratio” refers to the elongationratio that is determined from a tensile test performed on a JIS No. 5test piece sampled from a steel sheet with the tensile direction beingperpendicular to the rolling direction of the steel sheet.

It can be seen from FIG. 1 that the samples with good elongation eachshowed a gentle decrease in the quantity of retained austenite as theworking ratio increased.

Accordingly, we further measured the quantity of retained austenite ineach sample with TS of 780 MPa after subjection to tensile working withan elongation value of 10%, and examined the effect of the ratio of thequantity of retained austenite after the tensile working to the quantitybefore the tensile working on the total elongation of the steel sheet.The results are shown in FIG. 2.

It can be seen from FIG. 2 that elongation is good if the value obtainedby dividing the volume fraction of retained austenite remaining in asteel after subjection to tensile working with an elongation value of10% by the volume fraction of retained austenite before the tensileworking equals 0.3 or more, but otherwise elongation is poor.

Therefore, it is preferable in the disclosure that the value obtained bydividing the volume fraction of retained austenite remaining in a steelafter subjection to tensile working with an elongation value of 10% bythe volume fraction of retained austenite before the tensile workingequals 0.3 or more. The reason is that this set up may ensure thetransformation of sufficient retained austenite to martensite after theworking ratio becomes high enough.

The above-described TRIP phenomenon requires retained austenite to bepresent before performing press forming or working. Such retainedaustenite is a phase that remains when the Ms point (martensitetransformation start temperature), which depends on the elementscontained in the steel microstructure, is as low as approximately 15° C.or lower.

Specifically, in the tensile working with an elongation value of 10%according to the disclosure, a tensile test is performed on a JIS No. 5test piece sampled from a steel sheet with the tensile direction beingperpendicular to the rolling direction of the steel sheet, and the testis interrupted when the elongation ratio reaches 10%.

The volume fraction of retained austenite can be determined in theabove-described way.

Upon a detailed study of samples satisfying the above conditions, wediscovered that a TRIP phenomenon providing high strain hardenabilityoccurs upon working and even better elongation can be achieved if the Ccontent and the Mn content in the retained austenite satisfy thefollowing relation:

0.09*[Mn content]−0.026−0.150≦[C content]≦0.09*[Mn content]−0.026+0.150

where

[C content] is the C content in the retained austenite in mass %, and

[Mn content] is the Mn content in the retained austenite in mass %.

When the above requirements are met, it is possible to cause atransformation induced plasticity (TRIP) phenomenon, which is a keyfactor of improving ductility, to occur intermittently up until thefinal stage of working performed on the steel sheet, guaranteeing thegeneration of so-called stable retained austenite.

The C content in the retained austenite (in mass %) can be determined inthe following way.

Specifically, an EPMA is used to quantify the distribution of C in eachphase in a cross section along the rolling direction at a position ofsheet thickness×¼. Then, 30 retained austenite grains are analyzed todetermine respective C contents, the results are averaged, and theaverage is used as the C content. Note that the Mn content in theretained austenite (in mass %) can be determined in the same way as theC content in the retained austenite.

The following describes the production conditions.

Steel Slab Heating Temperature: 1100° C. or Higher and 1300° C. or Lower

Precipitates that are present at the time of heating of a steel slab(hereinafter, also referred to simply as a “slab”) will remain as coarseprecipitates in the resulting steel sheet, making no contribution tostrength. Thus, remelting of any Ti- and Nb-based precipitates formedduring casting is required.In this respect, if a steel slab is heated at a temperature below 1100°C., it is difficult to cause sufficient dissolution of carbides, leadingto problems such as an increased risk of trouble during the hot rollingresulting from increased rolling load. Therefore, the steel slab heatingtemperature needs to be 1100° C. or higher.In addition, from the perspective of obtaining a smooth steel sheetsurface by scaling-off defects in the surface layer of the slab, such asblow hole generation, segregation, and the like, and reducing cracks andirregularities over the steel sheet surface, the steel slab heatingtemperature needs to be 1100° C. or higher.If the steel slab heating temperature exceeds 1300° C., however, scaleloss increases as oxidation progresses. Therefore, the steel slabheating temperature needs to be 1300° C. or lower. For this reason, thesteel slab heating temperature is 1100° C. or higher and 1300° C. orlower. The steel slab heating temperature is preferably 1150° C. orhigher. The steel slab heating temperature is preferably 1250° C. orlower.

A steel slab is preferably made with continuous casting to prevent macrosegregation, yet may be produced with other methods such as ingotcasting or thin slab casting. The steel slab thus produced may be cooledto room temperature and then heated again according to a conventionalprocess. Moreover, energy-saving processes are applicable without anyproblem, such as hot direct rolling or direct rolling in which either awarm steel slab without being fully cooled to room temperature ischarged into a heating furnace, or a steel slab is hot rolledimmediately after being subjected to heat retaining for a short period.A steel slab is subjected to rough rolling under normal conditions andformed into a sheet bar. When the heating temperature is low, it ispreferable to additionally heat the sheet bar using a bar heater or thelike prior to finish rolling, from the viewpoint of preventing troublesduring the hot rolling.

Finisher Delivery Temperature in Hot Rolling: 750° C. or Higher and1000° C. or Lower

The heated steel slab is hot rolled through rough rolling and finishrolling to form a hot-rolled sheet. At this point, when the finisherdelivery temperature exceeds 1000° C., the amount of oxides (scales)generated suddenly increases and the interface between the steelsubstrate and oxides becomes rough, which tends to lower the surfacequality of the steel sheet after subjection to pickling and coldrolling. In addition, any hot rolling scales persisting after picklingadversely affect the ductility and stretch flangeability of the steelsheet. Moreover, grain size is excessively coarsened, causing surfacedeterioration in a pressed part during working. On the other hand, ifthe finisher delivery temperature is below 750° C., rolling loadincreases and rolling is performed more often with austenite being in anon-recrystallized state. As a result, an abnormal texture develops inthe steel sheet, and the final product has a significant planaranisotropy such that the material properties not only become lessuniform (the stability as a material decreases), but the ductilityitself also deteriorates.Therefore, the finisher delivery temperature in the hot rolling needs tobe 750° C. or higher and 1000° C. or lower. The finisher deliverytemperature is preferably 800° C. or higher. The finisher deliverytemperature is preferably 950° C. or lower.

Mean Coiling Temperature after Hot Rolling: 300° C. or Higher and 750°C. or Lower

When the mean coiling temperature after the hot rolling is above 750°C., the grain size of ferrite in the microstructure of the hot-rolledsheet increases, making it difficult to ensure a desired strength of thefinal-annealed sheet. On the other hand, when the mean coilingtemperature after the hot rolling is below 300° C., there is an increasein the strength of the hot-rolled sheet and in the rolling load for coldrolling, and the steel sheet suffers malformation. As a result,productivity decreases. Therefore, the mean coiling temperature afterthe hot rolling needs to be 300° C. or higher and 750° C. or lower. Themean coiling temperature is preferably 400° C. or higher. The meancoiling temperature is preferably 650° C. or lower.

According to the disclosure, finish rolling may be performedcontinuously by joining rough-rolled sheets during the hot rolling.Rough-rolled sheets may be coiled on a temporary basis. At least part offinish rolling may be conducted as lubrication rolling to reduce therolling load during the hot rolling. Conducting lubrication rolling insuch a manner is effective from the perspective of making the shape andmaterial properties of the steel sheet uniform. In lubrication rolling,the coefficient of friction is preferably 0.10 or more. The coefficientof friction is preferably 0.25 or less.

The hot-rolled sheet thus produced is subjected to pickling. Picklingenables removal of oxides from the steel sheet surface, and is thusimportant to ensure that the high-strength steel sheet as the finalproduct has good chemical convertibility and sufficient coating quality.The pickling may be performed in one or more batches.

Hot Band Annealing (Heat Treatment): To Retain in a Temperature Range ofAc₁ Transformation Temperature+20° C. to Ac₁ TransformationTemperature+120° C. for 600 s to 21,600 s

In this disclosure, it is very important to retain the steel sheet in atemperature range of Ac₁ transformation temperature+20° C. to Ac₁transformation temperature+120° C. for 600 s to 21,600 s.If the hot band annealing is performed at an annealing temperature belowAc₁ transformation temperature+20° C. or above Ac₁ transformationtemperature+120° C., or if the holding time is shorter than 600 s,concentration of Mn in austenite does not proceed in either case, makingit difficult to ensure a sufficient volume fraction of retainedaustenite after the final annealing. As a result, ductility decreases.On the other hand, if the steel sheet is retained for more than 21,600s, concentration of Mn in austenite reaches a plateau, and becomes lesseffective in improving ductility after the final annealing, resulting ina rise in costs.Therefore, in the hot band annealing (heat treatment) according to thedisclosure, the steel sheet is retained in a temperature range of Actransformation temperature+20° C. to Ac₁ transformation temperature+120°C. for 600 s to 21,600 s.

The above-described heat treatment process may be continuous annealingor batch annealing. After the above-described heat treatment, the steelsheet is cooled to room temperature. The cooling process and coolingrate are not particularly limited, however, and any type of cooling maybe performed, including furnace cooling and air cooling in batchannealing and gas jet cooling, mist cooling, and water cooling incontinuous annealing. The pickling may be performed according to aconventional process.

Rolling Reduction in Cold Rolling: 30% or More

The cold rolling according to the disclosure is performed at a rollingreduction of 30% or more. By performing the cold rolling at a rollingreduction of 30% or more, fine austenite is formed during heattreatment. As a result, fine retained austenite and martensite areformed in the steel sheet, improving the balance between strength andductility. The bendability and stretch flangeability (hole expansionformability) of the steel sheet are also improved.No upper limit is placed on the rolling reduction in the cold rollingaccording to the disclosure, yet a preferred upper limit is around 85%from the perspective of preventing excessive cold rolling load.

Cold-Rolled Sheet Annealing (Heat Treatment): To Retain in a TemperatureRange of Ac₁ Transformation Temperature to Ac₁ TransformationTemperature+100° C. for 20 s to 900 s

In this disclosure, it is very important to retain the steel sheet in atemperature range of Ac₁ transformation temperature to Ac₁transformation temperature+100° C. for 20 s to 900 s. When the annealingtemperature at which the cold-rolled sheet is annealed is below Ac₁transformation temperature or above Ac₁ transformation temperature+100°C., or if the holding time is shorter than 20 s, concentration of Mn inaustenite does not proceed in either case, making it difficult to ensurea sufficient volume fraction of retained austenite. As a result,ductility decreases. On the other hand, if the steel sheet is retainedfor more than 900 s, the area ratio of non-crystallized ferritedecreases and the interfaces between different phases, namely, betweenferrite and hard secondary phases (martensite and retained austenite),are reduced, leading to a reduction in both YP and YR.

Hot-Dip Galvanizing Treatment

In hot-dip galvanizing treatment according to the disclosure, the steelsheet subjected to the above-described cold-rolled sheet annealing (heattreatment) is dipped in a galvanizing bath at 440° C. or higher and 500°C. or lower for hot-dip galvanizing. Subsequently, the coating weight onthe steel sheet surface is adjusted using gas wiping or the like.Preferably, the hot-dip galvanizing is performed using a galvanizingbath containing 0.10 mass % or more and 0.22 mass % or less of Al.

Moreover, when a hot-dip galvanized layer is subjected to alloyingtreatment, the alloying treatment may be performed in a temperaturerange of 450° C. to 600° C. after the above-described hot-dipgalvanizing treatment. If the alloying treatment is performed at atemperature above 600° C., untransformed austenite transforms topearlite, where a desired volume fraction of retained austenite cannotbe ensured and ductility degrades. On the other hand, if the alloyingtreatment is performed at a temperature below 450° C., the alloyingprocess does not proceed, making it difficult to form an alloy layer.

Therefore, when the galvanized layer is subjected to alloying treatment,the alloying treatment is performed in a temperature range of 450° C. to600° C.

Although other manufacturing conditions are not particularly limited,the series of processes including the annealing, hot-dip galvanizing,and alloying treatment described above may preferably be performed in acontinuous galvanizing line (CGL), which is a hot-dip galvanizing line,from the perspective of productivity.

When hot-dip aluminum coating treatment is performed, the steel sheetsubjected to the above-described annealing treatment is dipped in analuminum molten bath at 660° C. to 730° C. for hot-dip aluminum coatingtreatment. Subsequently, the coating weight is adjusted using gas wipingor the like. If the steel sheet has a composition such that thetemperature of the aluminum molten bath falls within the temperaturerange of Ac₁ transformation temperature to Ac₁ transformationtemperature+100° C., the steel sheet is preferably subjected to hot-dipaluminum coating treatment because finer and more stable retainedaustenite can be formed, and therefore further improvement in ductilitycan be achieved.

According to the disclosure, electrogalvanizing treatment may also beperformed. No particular limitations are placed on theelectrogalvanizing treatment conditions, yet the electrogalvanizingtreatment conditions are preferably set so that the plated layer has athickness of 5 μm to 15 μm.

According to the disclosure, the above-described “high-strength steelsheet,” “high-strength hot-dip galvanized steel sheet,” “high-strengthhot-dip aluminum-coated steel sheet,” and “high-strengthelectrogalvanized steel sheet” may be subjected to skin pass rolling forthe purposes of straightening, adjustment of roughness on the sheetsurface, and the like. The skin pass rolling is preferably performed ata rolling reduction of 0.1% or more. The skin pass rolling is preferablyperformed at a rolling reduction of 2.0% or less.

When the rolling reduction is less than 0.1%, the skin pass rollingbecomes less effective and more difficult to control. Thus, a preferablerange for the rolling reduction has a lower limit of 0.1%. On the otherhand, when the skin pass rolling is performed at a rolling reductionabove 2.0%, the productivity of the steel sheet decreases significantly.Thus, the preferable range for the rolling reduction has an upper limitof 2.0%.

The skin pass rolling may be performed on-line or off-line. Skin passmay be performed in one or more batches to achieve a target rollingreduction.

Moreover, the “high-strength steel sheet,” “high-strength hot-dipgalvanized steel sheet,” “high-strength hot-dip aluminum-coated steelsheet,” and “high-strength electrogalvanized steel sheet” according tothe disclosure may be subjected to a variety of coating treatmentoptions, such as those using coating of resin, fats and oils, and thelike.

Examples

Steels having the chemical compositions as presented in Table 1, withthe balance consisting of Fe and incidental impurities, were prepared bysteelmaking in a converter, and formed into slabs through continuouscasting. The slabs thus obtained were formed into a variety of steelsheets, as described below, by varying the conditions as listed in Table2.

After being hot rolled, each steel sheet was annealed in a temperaturerange of Ac₁ transformation temperature+20° C. to Ac₁ transformationtemperature+120° C. After being cold rolled, each steel sheet wasannealed in a temperature range of Ac₁ transformation temperature to Ac₁transformation temperature+100° C. Consequently, a high-strengthcold-rolled steel sheet (CR) was obtained, and subjected to galvanizingtreatment to form a hot-dip galvanized steel sheet (GI), a galvannealedsteel sheet (GA), a hot-dip aluminum-coated steel sheet (Al), anelectrogalvanized steel sheet (EG), or the like.Used as hot-dip galvanizing baths were a zinc bath containing 0.19 mass% of Al for hot-dip galvanized steel sheets (GI) and a zinc bathcontaining 0.14 mass % of Al for galvannealed steel sheets (GA). Ineither case, the bath temperature was 465° C. and the coating weight perside was 45 g/m² (in the case of both-sided coating). For GA, the Feconcentration in the coating layer was adjusted to be 9 mass % or moreand 12 mass % or less. The bath temperature of the hot-dip aluminummolten bath for hot-dip aluminum-coated steel sheets was set at 700° C.For each of the steel sheets thus obtained, the cross-sectionalmicrostructure, tensile property, hole expansion formability,bendability, and the like were investigated. The results are listed inTables 3 and 4.

The Ac₁ transformation temperature was calculated by:

Ac₁transformation temperature(° C.)=

751−16*(% C)+11*(% Si)−28*(% Mn)−5.5*(% Cu)−16*(% Ni)+13*(% Cr)+3.4*(%Mo)

where (% C), (% Si), (% Mn), (% Ni), (% Cu), (% Cr), and (% Mo) eachrepresent the content in steel (in mass %) of the element in theparentheses.

Tensile test was performed in accordance with JIS Z 2241 (2011) tomeasure YP, YR, TS, and EL using JIS No. 5 test pieces, each of whichwas sampled in a manner that the tensile direction was perpendicular tothe rolling direction of the steel sheet. Note that YR is YP divided byTS, expressed as a percentage. In this case, the results were determinedto be good when YR≧68% and when TS*EL≧24,000 MPa·%. Also, EL wasdetermined to be good when EL 34% for TS 590 MPa grade, EL 30% for TS780 MPa grade, and EL 24% for TS 980 MPa grade. In this case, a steelsheet of TS 590 MPa grade refers to a steel sheet with TS of 590 MPa ormore and less than 780 MPa, a steel sheet of TS 780 MPa grade refers toa steel sheet with TS of 780 MPa or more and less than 980 MPa, and asteel sheet of TS 980 MPa grade refers to a steel sheet with TS of 980MPa or more and less than 1180 MPa.

Bend test was performed according to the V-block method specified in JISZ 2248 (1996). Each steel sheet was visually observed under astereoscopic microscope for cracks on the outside of the bent portion,and the minimum bending radius without cracks was used as the limitbending radius R. In this case, the bendability of the steel sheet wasdetermined to be good if the following condition was satisfied: limitbending radius R at 90° V-bending/t≦1.5 (where t is the thickness of thesteel sheet).

Hole expansion test was performed in accordance with JIS Z 2256 (2010).Each of the steel sheets obtained was cut to a size of 100 mm*100 mm,and a hole of 10 mm in diameter was drilled through each sample withclearance 12%±1%. Then, each steel sheet was clamped into a die havingan inner diameter of 75 mm with a blank holding force of 9 tons (88.26kN). In this state, a conical punch of 60° was pushed into the hole, andthe hole diameter at the crack initiation limit was measured. Then, toevaluate hole expansion formability, the maximum hole expansion ratio(%) was calculated by:

Maximum hole expansion ratio λ(%)={(D _(f) −D ₀)/D ₀}*100

where D_(f) is a hole diameter at the time of occurrence of cracking(mm) and D₀ is an initial hole diameter (mm).In this case, the maximum hole expansion ratio was determined to be goodwhen λ≧30% for TS 590 MPa grade, λ≧25% for TS 780 MPa grade, and λ≧20%for TS 980 MPa grade.

The sheet passage ability during hot rolling was determined to be lowwhen it was considered that the risk of troubles, such as malformationduring hot rolling due to increased rolling load, would increasebecause, for example, the hot-rolling finisher delivery temperature waslow and rolling would be performed more often with austenite being in anon-crystallized state, or rolling would be performed in anaustenite-ferrite dual phase region.

Similarly, the sheet passage ability during cold rolling was determinedto be low when it was considered that the risk of troubles, such asmalformation during cold rolling due to increased rolling load, wouldincrease because, for example, the coiling temperature during hotrolling was low and the hot-rolled sheet had a steel microstructure inwhich low-temperature transformation phases, such as bainite andmartensite, were dominantly present.

The surface characteristics of each final-annealed sheet were determinedto be poor when defects such as blow hole generation and segregation onthe surface layer of the slab could not be scaled-off, cracks andirregularities on the steel sheet surface increased, and a smooth steelsheet surface could not be obtained. The surface characteristics of eachfinal-annealed sheet were also determined to be poor when the amount ofoxides (scales) generated suddenly increased, interfaces between thesteel substrate and oxides were roughened, and the surface quality afterpickling and cold rolling degraded, or when hot-rolling scales persistedat least in part after pickling.

In this case, productivity was evaluated according to the lead timecosts, including: (1) malformation of a hot-rolled sheet occurred; (2) ahot-rolled sheet requires straightening before proceeding to thesubsequent steps; and (3) a prolonged holding time during the annealingtreatment. The productivity was determined to be “high” when none of (1)to (3) applied and “low” when any of (1) to (3) applied.

A value was obtained by dividing the volume fraction of retainedaustenite remaining in each steel sheet after subjection to 10% tensileworking by the volume fraction of retained austenite before the working(10%). The volume fraction of retained austenite was measured inaccordance with the above procedure.

The measurement results are also listed in Table 3.

The C content in the retained austenite (in mass %) and the Mn contentin the retained austenite (in mass %) were measured in accordance withthe above procedure.

The measurement results are also listed in Table 3.

TABLE 1 Steel Chemical Composition (mass %) ID C Si Mn P S N Ti Al Nb BNi Cr V Mo Cu A 0.103 0.34 3.23 0.021 0.0019 0.0035 0.032 — — — — — — —— B 0.162 0.64 3.82 0.026 0.0022 0.0037 0.038 — — — — — — — — C 0.1681.25 4.01 0.029 0.0021 0.0034 0.042 — — — — — — — — D 0.072 1.56 3.980.022 0.0019 0.0041 0.032 — — — — — — — — E 0.145 0.57 3.78 0.027 0.00180.0042 0.021 — — — — — — — — F 0.138 0.03 3.54 0.021 0.0027 0.0031 0.009— — — — — — — — G 0.102 0.87 3.95 0.027 0.0021 0.0034 0.022 — — — — — —— — H 0.104 0.67 2.87 0.026 0.0021 0.0034 0.032 — — — — — — — — I 0.1030.49 3.67 0.021 0.0026 0.0033 0.034 — — — — — — — — J 0.012 0.49 3.820.029 0.0026 0.0032 0.037 — — — — — — — — K 0.202 4.22 3.49 0.028 0.00270.0034 0.024 — — — — — — — — L 0.186 0.67 2.38 0.024 0.0029 0.0035 0.025— — — — — — — — M 0.172 0.61 3.81 0.025 0.0024 0.0035 0.001 — — — — — —— — N 0.204 0.33 3.82 0.022 0.0026 0.0038 0.032 0.42 — — — — — — — O0.182 0.89 3.67 0.028 0.0027 0.0034 0.028 — 0.044 — — — — — — P 0.1920.83 3.64 0.027 0.0023 0.0037 0.019 — — 0.0015 — — — — — Q 0.233 1.223.59 0.028 0.0022 0.0032 0.029 — — — 0.282 — — — — R 0.133 0.35 4.010.031 0.0022 0.0032 0.027 — — — — 0.344 — — — S 0.133 0.69 3.76 0.0280.0022 0.0032 0.023 — — — — — 0.038 — — T 0.129 0.54 3.39 0.025 0.00250.0036 0.035 — — — — — — 0.331 — U 0.102 1.45 3.12 0.031 0.0026 0.00350.042 — — — — — — — 0.274 V 0.109 0.56 3.63 0.026 0.0032 0.0034 0.041 —— — — — — — — W 0.121 0.59 3.19 0.027 0.0022 0.0035 0.035 — — — — — — —— X 0.198 0.69 3.59 0.029 0.0017 0.0044 0.039 — 0.047 — — — — — — Y0.204 0.43 3.23 0.027 0.0027 0.0037 0.037 — 0.032 — — — — — — Z 0.2130.29 3.76 0.026 0.0025 0.0044 0.034 — 0.038 — — — — — — AA 0.214 0.974.02 0.031 0.0028 0.0045 0.036 — — — — — — — — AB 0.199 1.28 3.85 0.0320.0029 0.0041 0.034 — — — — — — — — AC 0.195 1.24 4.12 0.029 0.00240.0033 0.026 — — — — — — — — AD 0.155 0.65 3.82 0.028 0.0020 0.00380.029 — — — — — — — — Ac₁ transfor- mation Chemical Composition temper-Steel (mass %) ature ID Sn Sb Ta Ca Mg REM (° C.) Remarks A — — — — — —663 Conforming Steel B — — — — — — 648 Conforming Steel C — — — — — —650 Conforming Steel D — — — — — — 656 Conforming Steel E — — — — — —649 Conforming Steel F — — — — — — 650 Conforming Steel G — — — — — —647 Conforming Steel H — — — — — — 675 Conforming Steel I — — — — — —651 Conforming Steel J — — — — — — 677 Comparative Steel K — — — — — —724 Comparative Steel L — — — — — — 689 Comparative Steel M — — — — — —648 Comparative Steel N — — — — — — 644 Conforming Steel O — — — — — —655 Conforming Steel P — — — — — — 655 Conforming Steel Q — — — — — —656 Conforming Steel R — — — — — — 643 Conforming Steel S — — — — — —650 Conforming Steel T — — — — — — 660 Conforming Steel U — — — — — —675 Conforming Steel V 0.006 — — — — — 652 Conforming Steel W — — 0.005— — — 665 Conforming Steel X — — — — — — 655 Conforming Steel Y 0.007 —— — — — 662 Conforming Steel Z — — 0.008 — — — 646 Conforming Steel AA —— — 0.0023 — — 646 Conforming Steel AB — — — — 0.0021 — 654 ConformingSteel AC — — — — — 0.0025 646 Conforming Steel AD — 0.012 — — — — 649Conforming Steel Underlined if outside of the disclosed range.

TABLE 2 Slab Finisher Mean Heat treatment of hot-rolled sheet RollingCold-rolled sheet annealing heating delivery coiling Heat treat- Heattreat- reduction in Heat treat- Heat treat- Steel temp. temp. temp. menttemp. ment time cold rolling ment temp. ment time No. ID (° C.) (° C.)(° C.) (° C.) (s) (%) (° C.) (s) Type* Remarks  1 A 1230 900 540 70318000 57.6 688 500 CR Example  2 B 1240 890 500 688 20000 54.8 673 300CR Example  3 C 1200 890 600 690 15000 52.9 675 350 GA Example  4 C 1230700 540 690 20000 47.1 675 250 CR Comparative Example  5 C 1240 1100 490690 10000 56.3 675 350 CR Comparative Example  6 C 1250 860 850 690 7000 56.5 675 800 CR Comparative Example  7 C 1270 870 510 500 1600064.7 675 700 EG Comparative Example  8 C 1240 880 490 850 17000 58.8 675500 CR Comparative Example  9 C 1270 890 570 690  300 50.0 675 600 CRComparative Example 10 C 1240 890 600 690 20000 52.9 675 550 CR Example11 C 1230 860 610 690 19000  9.1 653 650 CR Comparative Example 12 C1240 880 590 690  6000 57.1 520 750 CR Comparative Example 13 C 1210 880560 690 10000 51.7 850 400 Al Comparative Example 14 C 1230 860 560 69016000 58.8 675  2 CR Comparative Example 15 C 1200 890 570 690 2000058.8 675 1500  CR Comparative Example 16 D 1240 860 530 696 18000 58.8681 600 CR Example 17 E 1240 890 520 689  6000 57.1 674 700 GI Example18 F 1250 900 550 690 18000 50.0 675 500 CR Example 19 G 1240 890 590687  7000 52.9 672 550 Al Example 20 H 1260 860 560 715  9000 48.6 700540 CR Example 21 I 1240 920 600 691 15000 46.2 676 290 GA Example 22 J1220 860 630 717 17000 62.5 702 300 GI Comparative Example 23 K 1210 870620 764 20000 58.8 749 200 EG Comparative Example 24 L 1240 840 570 72919000 56.3 714 280 CR Comparative Example 25 M 1250 830 540 688  500062.5 673 360 EG Comparative Example 26 N 1260 850 580 684  6000 64.7 669370 GI Example 27 O 1270 870 540 695 15000 50.0 680 400 CR Example 28 P1220 900 520 695 18000 46.2 680 300 GA Example 29 Q 1260 840 600 69619000 52.9 681 320 CR Example 30 R 1270 830 560 683 14000 47.1 668 300EG Example 31 S 1240 880 620 690  8000 55.6 675 300 Al Example 32 T 1250820 600 700  8000 56.3 685 200 GI Example 33 U 1250 850 530 715 1500058.8 700 250 GI Example 34 V 1240 920 570 692 13000 70.6 677 280 GIExample 35 W 1230 910 500 705 10000 62.5 690 250 EG Example 36 X 1250890 590 695 16000 56.3 680 300 Al Example 37 Y 1260 900 520 702  900053.8 687 340 GA Example 38 Z 1250 880 540 686 18000 56.3 671 600 GIExample 39 AA 1260 900 520 686  9000 56.3 671 500 Al Example 40 AB 1250880 540 694 15000 56.3 679 500 CR Example 41 AC 1260 860 530 686  600060.0 671 350 CR Example 42 AD 1250 870 540 696  8000 53.3 692 400 CRExample 43 B 1230 890 550 695 10000 54.8 652 200 CR Example 44 B 1250870 530 698  8000 54.8 654 180 CR Example Underlined if outside of thedisclosed range. *CR: cold-rolled steel sheet (without coating orplating), GI: hot-dip galvanized steel sheet (without alloying treatmentof galvanized layer), GA: galvannealed steel sheet, Al: hot-dipaluminum-coated steel sheet, EG: electrogalvanized steel sheet

TABLE 3 Mean Mn content 0.09 × Mean Mean in (Mn Area Area Area VolumeMean Mean Mean Mn Mn RA/ content in Sheet ratio ratio ratio fractiongrain grain grain content content Mean RA) − thick- of of of of sizesize size in RA in F Mn 0.026 − Steel ness F F′ M RA of F of M of RA(mass (mass content 0.150 No. ID (mm) (%) (%) (%) (%) (μm) (μm) (μm) %)%) in F (mass %)  1 A 1.4 69.5 6.2  6.4 13.1 4.9 2.6 2.4 6.89 2.84 2.430.444  2 B 1.4 56.4 9.2 10.6 19.2 4.3 1.7 1.8 7.68 3.02 2.54 0.515  3 C1.6 42.5 16.9 13.5 24.6 3.2 1.0 1.1 8.22 3.08 2.67 0.564  4 C 1.8 58.610.2 12.8  7.1 3.9 1.5 1.6 7.45 2.79 2.67 0.495  5 C 1.4 60.8 10.8 13.4 7.4 4.1 1.8 1.4 7.35 2.89 2.54 0.486  6 C 1.0 56.9 10.3 12.5 18.6 7.84.2 4.1 6.89 2.97 2.32 0.444  7 C 1.2 60.4 10.4 15.4  6.4 5.4 1.8 1.75.41 3.57 1.52 0.311  8 C 1.4 60.9 12.1 16.8  6.9 5.2 1.5 1.4 5.32 3.671.45 0.303  9 C 1.2 58.9 11.4 14.9  6.2 5.1 1.6 1.5 5.26 3.68 1.43 0.29710 C 1.6 55.4  9.3 10.4 18.5 4.2 2.1 2.0 7.48 2.99 2.50 0.497 11 C 2.062.1 10.8 12.4  7.1 4.5 4.2 4.1 7.05 2.78 2.54 0.459 12 C 1.2 59.4 10.418.1  6.2 4.5 2.3 2.1 5.67 3.64 1.56 0.334 13 C 1.4 58.6 10.9 17.1  6.54.6 2.1 2.4 5.54 3.75 1.48 0.323 14 C 1.4 57.8 11.2 18.5  6.3 4.2 2.32.2 5.28 3.87 1.36 0.299 15 C 1.4 60.4  1.2  8.4 23.4 5.6 4.1 3.9 8.262.67 3.09 0.567 16 D 1.4 54.6  9.8 10.4 18.4 4.2 1.8 1.8 7.54 2.89 2.610.503 17 E 1.2 53.8 10.1 10.6 16.8 4.1 1.5 1.9 7.32 2.78 2.63 0.483 18 F1.4 53.2  9.8 10.9 19.2 3.9 1.9 1.8 7.84 2.97 2.64 0.530 19 G 1.6 71.4 6.8  6.9 12.8 5.1 2.7 2.3 6.95 2.85 2.44 0.450 20 H 1.8 71.5  6.5  7.213.8 4.8 2.5 2.2 7.08 2.76 2.57 0.461 21 I 1.4 70.8  6.9  7.5 13.5 4.62.4 2.4 6.85 2.66 2.58 0.441 22 J 1.2 85.2  6.4  3.8  3.6 7.4 0.6 0.56.45 2.79 2.31 0.405 23 K 1.4 50.4 18.4 15.4  6.8 5.4 3.9 3.8 7.29 2.872.54 0.480 24 L 1.4 62.4 13.2 15.9  6.2 5.8 4.5 3.9 3.40 2.08 1.63 0.13025 M 1.2 53.2  2.2 11.4 13.2 7.1 4.1 4.0 7.28 2.89 2.52 0.479 26 N 1.258.1 10.4 11.4 18.4 4.2 1.6 1.9 7.59 2.98 2.55 0.507 27 O 1.4 55.4 10.5 9.8 17.4 4.3 1.7 1.4 7.64 2.85 2.68 0.512 28 P 1.4 57.2 10.1 10.5 19.54.1 1.4 1.5 7.49 2.89 2.59 0.498 29 Q 1.6 56.2 10.3 10.8 19.6 4.3 1.71.6 7.39 2.98 2.48 0.489 30 R 1.8 70.5  6.8  7.5 12.8 5.1 2.5 2.3 6.582.75 2.39 0.416 31 S 1.6 71.5  6.7  8.2 12.4 4.8 2.4 2.1 6.87 2.81 2.440.442 32 T 1.4 68.9  6.5  7.2 14.2 4.6 2.3 2.1 6.89 2.74 2.51 0.444 33 U1.4 67.8  6.9  7.6 13.5 4.5 2.4 2.5 6.99 2.65 2.64 0.453 34 V 1.0 71.2 7.2  7.5 12.9 4.8 2.1 2.4 6.48 2.71 2.39 0.407 35 W 1.2 72.4  6.8  6.813.2 4.7 2.3 2.1 6.78 2.68 2.53 0.434 36 X 1.4 54.7 10.4 10.2 18.4 4.11.6 1.7 7.85 2.89 2.72 0.531 37 Y 1.2 58.2 10.5 10.6 16.9 4.5 1.7 1.87.46 2.79 2.67 0.495 38 Z 1.4 57.4  9.8  9.8 20.1 4.2 1.8 1.6 7.59 2.682.83 0.507 39 AA 1.4 55.8 10.6 10.6 19.4 3.9 1.9 1.5 7.36 2.89 2.550.486 40 AB 1.4 56.2 10.1  9.6 18.4 4.1 1.7 1.5 7.56 2.78 2.72 0.504 41AC 1.2 56.9  9.9 10.3 18.2 4.2 1.6 1.8 7.54 2.79 2.70 0.503 42 AD 1.454.0 11.1 12.1 19.6 3.9 1.4 1.6 7.94 2.91 2.73 0.539 43 B 1.4 53.4 10.111.2 18.4 4.5 1.9 1.7 7.24 3.24 2.23 0.476 44 B 1.4 54.1 10.5 10.8 19.14.6 1.8 1.8 7.18 3.28 2.19 0.470 Volume fraction of RA remaining 0.09 ×after 10% (Mn tensile content working in divided by RA) − C volume0.026 + content fraction 0.150 in RA of RA Residual (mass (mass beforethe micro- No. %) %) working structure Remarks  1 0.744 0.63 0.77 BF, P,θ Example  2 0.815 0.71 0.81 BF, P, θ Example  3 0.864 0.75 0.68 BF, P,θ Example  4 0.795 0.43 0.25 BF, P, θ Comparative Example  5 0.786 0.650.42 BF, P, θ Comparative Example  6 0.744 0.63 0.51 BF, P, θComparative Example  7 0.611 0.50 0.39 BF, P, θ Comparative Example  80.603 0.49 0.52 BF, P, θ Comparative Example  9 0.597 0.49 0.44 BF, P, θComparative Example 10 0.797 0.69 0.69 BF, P, θ Example 11 0.759 0.790.19 BF, P, θ Comparative Example 12 0.634 0.29 0.22 BF, P, θComparative Example 13 0.623 0.46 0.46 BF, P, θ Comparative Example 140.599 0.49 0.53 BF, P, θ Comparative Example 15 0.867 0.76 0.39 BF, P, θComparative Example 16 0.803 0.69 0.71 BF, P, θ Example 17 0.783 0.620.75 BF, P, θ Example 18 0.830 0.72 0.64 BF, P, θ Example 19 0.750 0.620.69 BF, P, θ Example 20 0.761 0.63 0.70 BF, P, θ Example 21 0.741 0.610.74 BF, P, θ Example 22 0.705 0.38 0.24 BF, P, θ Comparative Example 230.780 0.67 0.52 BF, P, θ Comparative Example 24 0.430 0.30 0.43 BF, P, θComparative Example 25 0.779 0.65 0.46 BF, P, θ Comparative Example 260.807 0.70 0.62 BF, P, θ Example 27 0.812 0.65 0.68 BF, P, θ Example 280.798 0.69 0.74 BF, P, θ Example 29 0.789 0.63 0.84 BF, P, θ Example 300.716 0.59 0.86 BF, P, θ Example 31 0.742 0.58 0.81 BF, P, θ Example 320.744 0.63 0.75 BF, P, θ Example 33 0.753 0.61 0.78 BF, P, θ Example 340.707 0.55 0.74 BF, P, θ Example 35 0.734 0.59 0.76 BF, P, θ Example 360.831 0.69 0.79 BF, P, θ Example 37 0.795 0.66 0.83 BF, P, θ Example 380.807 0.68 0.74 BF, P, θ Example 39 0.786 0.63 0.78 BF, P, θ Example 400.804 0.67 0.77 BF, P, θ Example 41 0.803 0.64 0.68 BF, P, θ Example 420.839 0.70 0.72 BF, P, θ Example 43 0.776 0.45 0.29 BF, P, θ Example 440.770 0.46 0.28 BF, P, θ Example Underlined if outside of the disclosedrange. F: polygonal ferrite, F′ : non-recrystallized ferrite, BF:bainitic ferrite, RA: retained austenite, M: martensite, P: pearlite, θ:carbides (such as cementite)

TABLE 4 Sheet Sheet Surface Sheet passage passage characteristics thick-ability ability of final- TS × EL Steel ness during during annealedProduc- YP YR TS EL MPa · λ No. ID (mm) hot rolling cold rolling sheettivity (MPa) (%) (MPa) (%) %) R/t (%) Remarks  1 A 1.4 High High GoodHigh 512 81.1 631 38.2 24104 0.2 61 Example  2 B 1.4 High High Good High689 85.1 810 35.1 28431 0.4 43 Example  3 C 1.6 High High Good High 99597.7 1018 31.4 31965 0.6 34 Example  4 C 1.8 Low Low Poor Low 694 83.2834 21.9 18265 1.7 19 Comparative Example  5 C 1.4 High Low Poor Low 68983.9 821 22.4 18390 2.1 20 Comparative Example  6 C 1.0 High High GoodHigh 498 90.7 549 28.4 15592 2.0 18 Comparative Example  7 C 1.2 HighLow Good High 697 82.7 843 22.5 18968 0.8 34 Comparative Example  8 C1.4 High High Good High 685 79.6 861 23.5 20234 0.7 35 ComparativeExample  9 C 1.2 High Low Good High 657 80.9 812 23.7 19244 0.8 32Comparative Example 10 C 1.6 High High Poor Low 646 76.4 846 35.9 303710.9 56 Example 11 C 2.0 High High Good High 654 79.7 821 21.6 17734 1.622 Comparative Example 12 C 1.2 High High Good High 697 80.7 864 21.518576 0.8 31 Comparative Example 13 C 1.4 High High Good Low 704 82.4854 20.6 17592 0.7 34 Comparative Example 14 C 1.4 High High Good High689 78.5 878 20.9 18350 0.7 35 Comparative Example 15 C 1.4 High HighGood High 486 59.1 822 36.2 29756 1.8 19 Comparative Example 16 D 1.4High High Good High 690 83.7 824 34.9 28758 0.4 40 Example 17 E 1.2 HighHigh Good High 657 77.3 850 32.8 27880 0.6 38 Example 18 F 1.4 High HighGood High 698 82.9 842 35.1 29554 0.4 41 Example 19 G 1.6 High High GoodHigh 512 79.8 642 37.8 24268 0.2 54 Example 20 H 1.8 High High Good High512 81.9 625 38.6 24125 0.1 62 Example 21 I 1.4 High High Good High 51282.4 621 38.9 24157 0.2 59 Example 22 J 1.2 High High Good High 320 58.3549 31.9 17513 0.2 61 Comparative Example 23 K 1.4 High High Poor High902 75.5 1194 15.8 18865 1.8 12 Comparative Example 24 L 1.4 High HighGood High 625 75.4 829 20.8 17243 1.1 37 Comparative Example 25 M 1.2High High Good High 498 59.1 842 28.9 24334 1.7 12 Comparative Example26 N 1.2 High High Good High 674 82.1 821 36.4 29884 0.4 38 Example 27 O1.4 High High Good High 659 77.4 851 35.8 30466 0.4 35 Example 28 P 1.4High High Good High 705 87.1 809 35.9 29043 0.5 41 Example 29 Q 1.6 HighHigh Good High 704 88.7 794 36.4 28902 0.3 40 Example 30 R 1.8 High HighGood High 502 80.8 621 39.4 24467 0.1 49 Example 31 S 1.6 High High GoodHigh 523 82.4 635 39.5 25083 0.2 58 Example 32 T 1.4 High High Good High500 83.6 598 41.2 24638 0.2 52 Example 33 U 1.4 High High Good High 53478.3 682 37.5 25575 0.2 60 Example 34 V 1.0 High High Good High 548 83.8654 36.9 24133 0.3 48 Example 35 W 1.2 High High Good High 540 81.7 66136.7 24259 0.2 50 Example 36 X 1.4 High High Good High 721 83.4 864 34.830067 0.4 38 Example 37 Y 1.2 High High Good High 705 82.6 854 35.129975 0.6 35 Example 38 Z 1.4 High High Good High 708 85.9 824 36.429994 0.4 36 Example 39 AA 1.4 High High Good High 678 83.2 815 36.930074 0.5 40 Example 40 AB 1.4 High High Good High 659 81.5 809 34.828153 0.4 41 Example 41 AC 1.2 High High Good High 700 85.0 824 35.229005 0.4 36 Example 42 AD 1.4 High High Good High 782 79.5 984 32.632078 0.4 32 Example 43 B 1.4 High High Good High 589 74.7 789 31.524854 0.7 33 Example 44 B 1.4 High High Good High 598 75.3 794 30.824455 0.7 31 Example

From above, it can be seen that the steel sheets according to thedisclosure each exhibited TS of 590 MPa or more and YR of 68% or more,and are thus considered as high-strength steel sheets having excellentformability and high yield ratio. In contrast, the comparative examplesare inferior in terms of at least one of YR, TS, EL, λ, or R/t.

INDUSTRIAL APPLICABILITY

According to the disclosure, it becomes possible to manufacturehigh-strength steel sheets with excellent formability and high yieldratio that exhibit TS of 590 MPa or more and YR of 68% or more and thatsatisfy the condition of TS*EL≧24,000 MPa·%. High-strength steel sheetsaccording to the disclosure are highly beneficial in industrial terms,because they can improve fuel efficiency when applied to, for example,automobile structural parts, by a reduction in the weight of automotivebodies.

1. A high-strength steel sheet comprising: a chemical compositioncontaining, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01%or more and 3.00% or less, Mn: 2.60% or more and 4.20% or less, P:0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% orless, N: 0.0005% or more and 0.0100% or less, and Ti: 0.005% or more and0.200% or less, and the balance consisting of Fe and incidentalimpurities; and a steel microstructure that contains, in area ratio, 35%or more and 80% or less of polygonal ferrite, 5% or more ofnon-recrystallized ferrite, and 5% or more and 25% or less ofmartensite, and that contains, in volume fraction, 8% or more ofretained austenite, wherein the polygonal ferrite has a mean grain sizeof 6 μm or less, the martensite has a mean grain size of 3 μm or less,the retained austenite has a mean grain size of 3 μm or less, and avalue obtained by dividing an Mn content in the retained austenite inmass % by an Mn content in the polygonal ferrite in mass % equals 2.0 ormore.
 2. The high-strength steel sheet according to claim 1, wherein thechemical composition further contains, in mass %, at least one selectedfrom the group consisting of Al: 0.01% or more and 2.00% or less, Nb:0.005% or more and 0.200% or less, B: 0.0003% or more and 0.0050% orless, Ni: 0.005% or more and 1.000% or less, Cr: 0.005% or more and1.000% or less, V: 0.005% or more and 0.500% or less, Mo: 0.005% or moreand 1.000% or less, Cu: 0.005% or more and 1.000% or less, Sn: 0.002% ormore and 0.200% or less, Sb: 0.002% or more and 0.200% or less, Ta:0.001% or more and 0.010% or less, Ca: 0.0005% or more and 0.0050% orless, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or moreand 0.0050% or less.
 3. The high-strength steel sheet according to claim1, wherein the retained austenite has a C content that satisfies thefollowing formula in relation to the Mn content in the retainedaustenite:0.09*[Mn content]−0.026−0.150≦[C content]≦0.09*[Mn content]−0.026+0.150where [C content] is the C content in the retained austenite in mass %,and [Mn content] is the Mn content in the retained austenite in mass %.4. The high-strength steel sheet according to claim 1, wherein when thesteel sheet is subjected to tensile working with an elongation value of10%, a value obtained by dividing a volume fraction of the retainedaustenite after the tensile working by a volume fraction of the retainedaustenite before the tensile working equals 0.3 or more.
 5. Thehigh-strength steel sheet according to claim 1, wherein thehigh-strength steel sheet is a high-strength hot-dip galvanized steelsheet comprising a hot-dip galvanized layer, a high-strength hot-dipaluminum-coated steel sheet comprising a hot-dip aluminum-coated layeror a high-strength electrogalvanized steel sheet comprising anelectrogalvanized layer.
 6. (canceled)
 7. (canceled)
 8. A method formanufacturing the high-strength steel sheet as recited in claim 1, themethod comprising: heating a steel slab having the chemical compositioncontaining, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01%or more and 3.00% or less, Mn: 2.60% or more and 4.20% or less, P:0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% orless, N: 0.0005% or more and 0.0100% or less, and Ti: 0.005% or more and0.200% or less, and the balance consisting of Fe and incidentalimpurities, to 1100° C. or higher and 1300° C. or lower; hot rolling thesteel slab with a finisher delivery temperature of 750° C. or higher and1000° C. or lower to obtain a steel sheet; coiling the steel sheet at300° C. or higher and 750° C. or lower; then subjecting the steel sheetto pickling to remove scales; retaining the steel sheet in a temperaturerange of Ac₁ transformation temperature+20° C. to Ac₁ transformationtemperature+120° C. for 600 s to 21,600 s; cold rolling the steel sheetat a rolling reduction of 30% or more; and then retaining the steelsheet in a temperature range of Ac₁ transformation temperature to Ac₁transformation temperature+100° C. for 20 s to 900 s, and subsequentlycooling the steel sheet.
 9. A method for manufacturing the high-strengthsteel sheet as recited in claim 5, the method comprising: heating asteel slab having the chemical composition containing, in mass %, C:0.030% or more and 0.250% or less, Si: 0.01% or more and 3.00% or less,Mn: 2.60% or more and 4.20% or less, P: 0.001% or more and 0.100% orless, S: 0.0001% or more and 0.0200% or less, N: 0.0005% or more and0.0100% or less, and Ti: 0.005% or more and 0.200% or less, and thebalance consisting of Fe and incidental impurities, to 1100° C. orhigher and 1300° C. or lower; hot rolling the steel slab with a finisherdelivery temperature of 750° C. or higher and 1000° C. or lower toobtain a steel sheet; coiling the steel sheet at 300° C. or higher and750° C. or lower; then subjecting the steel sheet to pickling to removescales; retaining the steel sheet in a temperature range of Ac₁transformation temperature+20° C. to Ac₁ transformation temperature+120°C. for 600 s to 21,600 s; cold rolling the steel sheet at a rollingreduction of 30% or more; then retaining the steel sheet in atemperature range of Ac₁ transformation temperature to Ac₁transformation temperature+100° C. for 20 s to 900 s, and subsequentlycooling the steel sheet; and then subjecting the steel sheet to any oneof the following: galvanizing treatment, either alone or followed byalloying treatment at 450° C. or higher and 600° C. or lower, hot-dipaluminum-coating treatment, or electrogalvanizing treatment. 10.(canceled)
 11. (canceled)
 12. The high-strength steel sheet according toclaim 2, wherein the retained austenite has a C content that satisfiesthe following formula in relation to the Mn content in the retainedaustenite:0.09*[Mn content]−0.026−0.150[C content]0.09*[Mn content]−0.026+0.150where [C content] is the C content in the retained austenite in mass %,and [Mn content] is the Mn content in the retained austenite in mass %.13. The high-strength steel sheet according to claim 2, wherein when thesteel sheet is subjected to tensile working with an elongation value of10%, a value obtained by dividing a volume fraction of the retainedaustenite after the tensile working by a volume fraction of the retainedaustenite before the tensile working equals 0.3 or more.
 14. Thehigh-strength steel sheet according to claim 2, wherein thehigh-strength steel sheet is a high-strength hot-dip galvanized steelsheet comprising a hot-dip galvanized layer, a high-strength hot-dipaluminum-coated steel sheet comprising a hot-dip aluminum-coated layeror a high-strength electrogalvanized steel sheet comprising anelectrogalvanized layer.
 15. A method for manufacturing thehigh-strength steel sheet as recited in claim 2, the method comprising:heating a steel slab having the chemical composition containing, in mass%, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 3.00% orless, Mn: 2.60% or more and 4.20% or less, P: 0.001% or more and 0.100%or less, S: 0.0001% or more and 0.0200% or less, N: 0.0005% or more and0.0100% or less, and Ti: 0.005% or more and 0.200% or less, and at leastone selected from the group consisting of Al: 0.01% or more and 2.00% orless, Nb: 0.005% or more and 0.200% or less, B: 0.0003% or more and0.0050% or less, Ni: 0.005% or more and 1.000% or less, Cr: 0.005% ormore and 1.000% or less, V: 0.005% or more and 0.500% or less, Mo:0.005% or more and 1.000% or less, Cu: 0.005% or more and 1.000% orless, Sn: 0.002% or more and 0.200% or less, Sb: 0.002% or more and0.200% or less, Ta: 0.001% or more and 0.010% or less, Ca: 0.0005% ormore and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, andREM: 0.0005% or more and 0.0050% or less, and the balance consisting ofFe and incidental impurities, to 1100° C. or higher and 1300° C. orlower; hot rolling the steel slab with a finisher delivery temperatureof 750° C. or higher and 1000° C. or lower to obtain a steel sheet;coiling the steel sheet at 300° C. or higher and 750° C. or lower; thensubjecting the steel sheet to pickling to remove scales; retaining thesteel sheet in a temperature range of Ac₁ transformation temperature+20°C. to Ac₁ transformation temperature+120° C. for 600 s to 21,600 s; coldrolling the steel sheet at a rolling reduction of 30% or more; and thenretaining the steel sheet in a temperature range of Ac₁ transformationtemperature to Ac₁ transformation temperature+100° C. for 20 s to 900 s,and subsequently cooling the steel sheet.
 16. A method for manufacturingthe high-strength steel sheet as recited in claim 14, the methodcomprising: heating a steel slab having the chemical compositioncontaining, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01%or more and 3.00% or less, Mn: 2.60% or more and 4.20% or less, P:0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% orless, N: 0.0005% or more and 0.0100% or less, and Ti: 0.005% or more and0.200% or less, and at least one selected from the group consisting ofAl: 0.01% or more and 2.00% or less, Nb: 0.005% or more and 0.200% orless, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or moreand 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% ormore and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb:0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% orless, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, and thebalance consisting of Fe and incidental impurities, to 1100° C. orhigher and 1300° C. or lower; hot rolling the steel slab with a finisherdelivery temperature of 750° C. or higher and 1000° C. or lower toobtain a steel sheet; coiling the steel sheet at 300° C. or higher and750° C. or lower; then subjecting the steel sheet to pickling to removescales; retaining the steel sheet in a temperature range of Ac₁transformation temperature+20° C. to Ac₁ transformation temperature+120°C. for 600 s to 21,600 s; cold rolling the steel sheet at a rollingreduction of 30% or more; and then retaining the steel sheet in atemperature range of Ac₁ transformation temperature to Ac₁transformation temperature+100° C. for 20 s to 900 s, and subsequentlycooling the steel sheet; and then subjecting the steel sheet to any oneof the following: galvanizing treatment, either alone or followed byalloying treatment at 450° C. or higher and 600° C. or lower, hot-dipaluminum-coating treatment, or electrogalvanizing treatment.